The use of stop and search powers by the police has always been controversial, with critics claiming that it is discriminatory and an infringement of civil liberties. Rather less attention is focused on whether it actually reduces crime. Last January, a year and a half after the Met increased its use of stop and search under section 60 of 1994 Criminal Justice and Public Order Act as part of Operation Blunt 2, criminologist Marian Fitzgerald compiled data demonstrating that any connection between the amount of section 60 stop-and-search and the amount of knife crime offences in London's boroughs was at best extremely hard to identify.

Fitzgerald, who worked for the Home Office under both Conservative and Labour governments, showed that between April and October 2009 the largest fall in knife crime in the ten boroughs where rates had been highest occurred in Islington, which also had the secondest fewest section 60 stop-and-searches. By contrast, Southwark saw knife crime rise by 8.6% despite the second highest number of stop-and-searches - 9,437 - being conducted there.

More recent Met figures about knife crime and stop-and-search suggest to me that any causal link between the two remains difficult to pin down. Met stats showing knife crime offences where the knife was used to injure classified by borough show that in 17 boroughs there had been an increase during February to April of this year compared with the previous three months, no change in two and a fall in 13.

I've compared the borough figures with those for the numbers of stop-and-searches conducted during the third and fourth quarters (Q3 and Q4) of financial year 2010/11 (Appendix 2 of this document, which was supplied to a Metropolitan Police Authority committee meeting held in June). The six-month periods the two sets of figures relate to don't precisely coincide - Q3 and Q4 cover October 2010 to March 2011 while the knife crime figures cover November 2010 to April 2011 - but there's a five month overlap, and the comparison clearly provides food for thought similar to that served up by Marian Fitzgerald.

For example, the number of searches in Hackney increased by nearly 1,000 in Q4 compared with Q3 - up from 2,296 to 3,245 - yet between November 1010-January 2011 and February 2011-April 2011 the number of knife crime injury victims too increased, albeit by only one (from 39 to 40). In Harrow, the number of searches rose from 125 to 209 and the number of knife crime injures rose from ten to 23. In Lewisham searches nearly doubled from 715 to 1,365 and knife crime injuries went up from 50 to 56.

There are also boroughs where, by contrast, the correlation was between an increase in stop and search and a fall in knife crime injuries. In Havering, for example, searches increased from 142 to 275 and knife crime injuries fell from 28 to 16. Meanwhile in Newham the respective figures are searches down from 5,871 to 3,804, and knife crime injures up from 57 to 66. To complicate the picture further, two boroughs - Sutton and Kensington and Chelsea - saw small falls in both searches and knife crime injuries.

The overall comparison shows a very mixed picture. In eleven boroughs stop and search and knife crime injuries both went up, and in four knife crime injures either fell or stayed the same at the same time as searches fell. In only seven did an increase in searches coincide with a fall in knife crime injuries, while in nine a reduction in stop and searches coincided with an increase in knife crime injuries.

The true significance of these figures is very difficult to be sure of. It can still be argued that without stop and search all the knife injury figures in every borough would have been higher - the case Boris Johnson effectively makes with his soundbite about taking 11,000 knives off the streets. Yet the opposite can still be argued too. And the figures hardly undermine the scepticism of those who find the value of stop and search questionable, especially as it uses up a lot of resources and if it creates barriers between the police and young men in London rather than building bridges of confidence between them - bridges that vital local intelligence could flow across.

One thing, though, seems clear enough. In February to April this year, the total number of recorded knife crime injuries for the whole of London went up from 941 to 1,070 compared with the previous three months. On Tuesday I reported that injured knife crime victims in the 13-24 aged group have increased by more than 30% during the period of Operation Blunt 2 and knife crimes of all kinds against the same age group by more than 20%. Whatever can be claimed in favour of Blunt 2, it cannot be said to have brought knife crime down. And if there's no clear link between more street searches and less knife crime, is the tactic really justified?